1. Field of the Invention
This invention relates generally to a bi-directional amplifier and, more particularly, to an asymmetric, voltage optimized, wideband common-gate bi-directional amplifier for a transceiver, where the amplifier is optimized for both low noise amplification and high power amplification.
2. Discussion of the Related Art
Spacecraft-based surveillance and communications systems, such as satellite based radar systems, generally employ phased antenna arrays that require large apertures, on the order of 100 square meters or greater, to achieve the high spatial resolution necessary to support various communications protocols, such as GMTI, AMTI and SAR, known to those skilled in the art. Current technology and design approaches that support this large of a space-based phased array are generally impractical and expensive. For example, a phased antenna array operating at X-band (6-12 GHz) having a square aperture with side dimensions of ten meters, and taking into account the mutual coupling between adjacent radiation elements, can include up to 300,000 circuit elements. A typical spacecraft allocation of 20 kW of power and 10,000 pounds of payload would require each circuit element to consume no more than 50-100 mW of DC power and weigh less this 10-20 grams. Hence, the power and weight requirements are severe at the component level.
Each channel of a phased antenna array for these types of applications typically employs a transceiver module that processes both the signals received by the system and the signals transmitted by the system at different frequency bands. Each transceiver module generally has two separate signal amplification paths, one including a high power amplifier (HPA) for the transmit signal and one including a low noise amplifier (LNA) for the receive signal. The LNA typically has higher gain than the HPA because the receive signal has a very low intensity that is close to the noise floor. Four separate monolithic millimeter integrated chips (MMIC) are generally required to accommodate the amplification paths in each channel, one for the LNA, one for the HPA, and two for routing switches to switch the signal path between the transmit signal and the receive signal. The routing switches may be relays for low frequency applications or semiconductor switches, such as high electron mobility transistors (HEMT) or heterojunction bipolar transistors (HBT), for high frequency applications.
In a volume production environment, the high part count of the phased array complicates the manufacturing process and usually leads to undesired module rework. Further, the signal routing switches in the RF path incur losses that degrade the output power and the noise figure of the system affecting its performance. For example, the routing switch in front of the LNA may cause the noise to grow, which may not allow the system to detect the receive signal above the noise. Switch losses of this type may be on the order of 1-1.5 dB.
It is desirable to minimize the number of parts in a transceiver module, especially in spacecraft-based applications. To attain this goal, it has heretofore been known in the art to employ bi-directional amplifiers in each channel of a transceiver module, where the bi-directional amplifier amplifies both the transmit signals and the receive signals propagating in opposite directions. Because a bi-directional amplifier is used in this application, the routing switches normally required to route the receive signal to the LNA and the transmit signal to the HPA can be eliminated.
U.S. Pat. No. 5,821,813 issued to Batchelor et al. Oct. 13, 1998 discloses a bi-directional amplifier for this purpose. The ""813 bi-directional amplifier employs a field effect transistor that is connected in a common gate mode with the common terminal of each port of the amplifier and with the gate of the transistor. The source and drain terminals of the transistor are connected to a corresponding one of the ports through an impedance matching device. However, the ""813 bi-directional amplifier provides the same level of signal gain for the transmit signal and the receive signal. Thus, this bi-directional amplifier is not separately optimized for the transmit signals and the receive signals, and thus does not provide the best performance.
In accordance with the teachings of the present invention, a bi-directional amplifier is disclosed that has particular application for use in a transceiver module for amplifying both transmit signals and receive signals propagating in opposite directions. The amplifier includes first and second common gate field effect transistors (FETs) electrically coupled along a common transmission line. A first variable matching network is electrically coupled to the transmission line between a transmit signal input port and the first FET, and a second variable matching network is electrically coupled to the transmission line between a receive signal input port and the second FET. An interstage variable matching network is electrically coupled to the transmission line between the first and second FETs. A DC voltage regulator provides a DC bias signal to the matching networks and the FETs so that different signal amplifications and different impedance matching characteristics can be provided for the transmit signal and the receive signal.